1. Field of the Invention
The present invention relates to an alignment film for aligning a liquid crystal layer interposed between two substrates, a composition for forming an alignment film for forming the alignment film, and a liquid crystal display device including the alignment film.
2. Description of the Related Art
Liquid crystal display devices have good characteristics that they have a high display quality, are thin and light, consume little electric power, and so on, therefore, the application thereof has been expanded. For example, liquid crystal display devices are applied to monitors for portable goods such as monitors for cellular phones and monitors for digital still cameras, monitors for desktop personal computers, monitors for printing and designing, monitors for medical use, liquid crystal televisions, and so on. As expanding the application thereof, there have been strong demands for increasing transmittance to increase brightness, decreasing power consumption, and also decreasing cost for liquid crystal display devices.
In general, display in a liquid crystal display device is carried out by applying an electric field to liquid crystal molecules in a liquid crystal layer interposed between a pair of substrates to change the alignment direction of the liquid crystal molecules, and utilizing a change in the optical characteristics of the liquid crystal layer caused by the change of the alignment direction. The alignment direction of liquid crystal molecules when an electric field is not applied is determined by an alignment film in which the surface of a polyimide thin film is subjected to a rubbing treatment. Conventionally, in an active drive type liquid crystal display device having a switching element such as a thin-film transistor (TFT) for each pixel, an electrode is provided on each of a pair of substrates interposing a liquid crystal layer therebetween, so that the direction of an electric field to be applied to the liquid crystal layer may be substantially perpendicular to the surface of the substrate (a so-called vertical electric field), and display is carried out by utilizing the optical rotation of the liquid crystal molecules constituting the liquid crystal layer. As a representative display method adopted in a vertical electric field mode liquid crystal display device, a twisted nematic (TN) mode is known.
In a liquid crystal display device adopting this TN mode, a narrow viewing angle is one of the big problems. Accordingly, as a display mode achieving a wide viewing angle, an in-plane switching (IPS) mode or a vertical alignment (VA) mode are known.
It is known that in several types of liquid crystal display devices, if the volume specific resistance of an alignment film is high, a residual charge remains in the liquid crystal display devices to cause an afterimage (image burn-in). JP 2007-241249 A proposes that an alignment film reduces the volume specific resistance in a VA mode.